For the power converters, usually the multi-stage circuit configurations would be employed when the reliability and the simplicity of production are under consideration. The aforementioned multi-stage configurations almost always have the DC Bus, and the DC Bus usually is coupled to a capacitor having a relatively high capacitance in parallel. For example, usually the two-stage configuration having a DC Bus would be employed in the power source of the communication system. Usually, the front-end is a power factor correction (PFC) circuit, the back-end is a DC-DC converter, and the DC Bus is coupled between the two stages. The PFC circuit of the front-end would turn the AC input voltage into a DC output voltage, and the DC output voltage is sent to the DC Bus. The DC-DC converter of the back-end would input the current from the DC Bus. When the circuit is working under a steady state, the mean value of the input currents of the DC Bus equals to the mean value of the output currents of the DC Bus, but the transient currents of the two are different. Which means a capacitor must be coupled to the DC Bus in parallel for allowing the AC current to pass only so as to balance the transient powers between the two stages.
In general, the capacitor occupies a relatively large volume in a power converter and the cost of the capacitor is relatively high. Due to the unique features of the capacitor, the temperature rising during its operations should be kept at a relatively lower level to endure its life span. There are two reasons for the temperature rising of the capacitor: 1. The AC current will have losses on the capacitor equivalent series resistance (ESR) resistor, and 2. The influences come from the temperature of the environments and the heat dissipating conditions, and the influences of the other heat-generating elements are also included. Since the power densities of the converters are rising, the relatively larger volume and the heat-dissipating problems of the capacitors have become more and more important. The operational status of the DC Bus capacitor in the power source of the traditional communication system and the heat-dissipating resolutions in the prior arts are described as follows.
Using the power source of the traditional communication system as an example, the operational status of the DC Bus capacitor is described firstly. Please refer to FIG. 1, it shows the schematic circuit diagram of the typical power source of the communication system. In which, the power factor correction (PFC) unit is composed by the input rectified voltage Vin, the inductor L, the power diode D, the power switch S, and the DC Bus capacitor Cl, The phase-shifted full-bridge DC-DC converter is composed by the DC Bus capacitor Cl, the power switches S1–S4, the capacitor Cb, the inductor Ls, the transformer, and the output rectifying and filtering unit. The detailed operational procedures of the PFC circuit and the phase-shifted full-bridge DC-DC converter are not discussed here. Among which, the frequency of the output power of the phase-shifted full-bridge DC-DC converter is twice of its switching frequency, and the frequency of the input current of this stage is also twice of its switching frequency.
In FIG. 1, Cl is the DC Bus capacitor, and it is usually a capacitor having a relatively large capacitance. Due to the differences between the frequencies and the amplitudes of the output current iin of the front-end and the input current io of the back-end, there is an AC current IC which flows through the capacitor Cl. As for the power-level, the output power of the back-end is a constant, which means that the input current io mainly has the DC component except for the high frequency component. Theoretically, the output current of the front-end PFC unit mainly includes the 100 HZ component, and the high switching-frequency component is also included. The difference between the currents of the output of the front-end and the input of the back-end is the DC Bus capacitor current, which mainly has the 100 Hz component and the component around the switching frequency. When the input/output conditions of the converter are fixed, the 100 HZ component of the capacitor current is also fixed.
Assume that the input voltage of the PFC unit is 176 Vrms, the switching frequency is 45 KHZ, the DC Bus voltage is 360V, and the input current of the back-end is 8.33 Apk (7.5 A in average), the duty ratio is 0.9, the output voltage is 54V, the output current is 50 A, and the switching frequency is 80 KHZ. These pre-assumptions are set for the typical 3000 W/48 V/50 A power source of the communication system, which is operated under the poor operational conditions, and has certain representative features.
According to the above-mentioned pre-assumptions, the steady state waveforms of the DC Bus capacitor current are shown in FIGS. 2(a)–2(c). In which, the left-hand side waveform diagrams are the partial enlargements of the right-hand side operational frequency versus time period diagrams respectively. The waveform as shown in FIG. 2(a) is the waveform of the current flowing from the diode to the DC Bus, iin, and the envelope of which is a sinusoidal half-wave. The average value of the current iin is equal to the average value of the current io (the waveform of io is shown in FIG. 2(b)). The difference between iin and iO is shown in FIG. 2(c). In which, there are high frequency current ripples and the 100 HZ current components, and there is no DC component theoretically. When the load of the back-end is relatively large, the high frequency ripples are relatively large since the current flowing from the front-end to the DC Bus is a pulse wave and the amplitudes of the pulse wave are relatively larger.
Please refer to FIG. 3, it shows the frequency spectrum analysis diagram of the current flowing through the DC Bus capacitor (see FIG. 2(c)). In which, there are high frequency components around the front-end switching frequency. The effective value of the current of 100 HZ is 5.304 A, the effective value of the current above 100 HZ is 7.749 A, and the total effective value of the current is 9.389 A. When the high frequency component of the current is relatively too high, the effective value of the total current will be increased.
In the prior art, in order to solve the heat-dissipating problem, usually the number or the volume of the capacitor is increased to decrease the capacitor ESR resistor so as to decrease the losses. This method is quite simple, but the volume of the system is increased, and the power density of the converter is decreased. The other alternative is to increase the heat-dissipating capability of the capacitor, for example, increase the wind-force of the system etc. The second method has certain effects, but it is limited when the system is operated under the relatively high-temperature environments.
The present invention resolves the heat-dissipating problem of the DC Bus capacitor through decreasing the heat generated. If the current flowing through the capacitor can be decreased effectively, the heat generated by the capacitor would be decreased.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the DC-DC converter circuits and the method for reducing the DC Bus capacitor current are finally conceived by the applicants.